The present invention relates to the field of incremental encoders, i.e. devices which monitor the angular displacement of a rotating shaft. The invention also provides a means of generating commutation signals necessary for controlling the operation of brushless motors.
Various methods of measuring angular displacements of motor shafts have been proposed in the prior art. For example, U.S. Pat. No. 3,745,544 discloses an apparatus for digitally measuring the angle of rotation of a shaft. The device uses a set of rotating disks attached to the shaft, the disks having slotted portions which are used to produce information on angular position. The outputs of a plurality of magnetic sensors are passed through digital logic circuitry to produce the desired information.
A frequently used method of determining the angular position of a shaft is directing a light beam at a slotted disk, thereby causing a series of pulses to be generated. U.S. Pat. No. 4,328,463 shows an example of an arrangement of this type. Another system for measuring the angular position of a shaft is described in U.S. Pat. No. 4,443,787, which also uses a light-driven device to derive its basic measurement of a reference position of the shaft.
It has also been known to generate a series of digital pulses, corresponding to an analog signal which represents displacement of a shaft, and to sum those pulses to obtain a measurement of absolute angular displacement. An example of such a technique is given in U.S. Pat. No. 4,072,893.
The measurement of angular displacement of a shaft is typically based on two phase-shifted signals representing the motion of the shaft. When these signals are 90.degree. out of phase, the signals are called "quadrature" signals. The quadrature signals, taken together, encode both the angular displacement and the direction of motion of the shaft. Methods for determining displacement and direction of motion, from a train of quadrature pulses, are well known in the art, as exemplified by U.S. Pat. Nos. 4,157,507 and 4,442,532. U.S. Pat. No. 4,220,924 discloses another circuit for processing a pairS of quadrature-phased digital signals.
Encoders of the prior art, which typically use optical disks for generating a pulse train, have certain disadvantages. The rotating disks are mechanical devices which are prone to malfunction. The light beams used to generate signals must be precisely aligned, and can be easily disturbed by unwanted vibrations or other movements. An optical system is not reliable in a dusty enviroment, because the dust clouds the light sensors. The brightness of the light emitting diodes, commonly used in optical systems, is likely to decay over time. The present invention provides a system which has no moving parts, other than the shaft and armature magnets themselves. This construction adds considerably to the ruggedness and reliability of the system.
The present invention also provides commutation signals necessary for the control of a brushless motor. Brushless motor windings are usually commutated according to a fixed timing scheme. That is, over a 360.degree. cycle, the various armature windings are energized and deenergized with a fixed timing sequence. With this scheme, however, the torque of the motor decreases at higher rotational speeds. This reduction in torque is believed to be due to the phase delay caused by the nonzero inductance and resistance of the motor windings. As the motor speed increases, the frequency of the commutation signals increases, and the inductance of the windings becomes more significant. The commutation signal is, in effect, delayed relative to its proper timing.
One can compensate for this delay by advancing the commutation signals by a certain angular displacement, depending on the motor speed. For example, the amount of commutation advance can be made directly proportional to the speed of the motor. The effective speed range of the motor is thereby increased, because the motor does not lose torque at higher rotational velocities.
The present invention provides means for generating commutation pulses, for various amounts of advance or retard. The present invention therefore has the additional advantage that it provides both quadrature pulses and commutation pulses in the same apparatus. The invention can therefore be used both to monitor the angular displacement and direction of rotation of a shaft, and to control effectively the brushless motor which drives that shaft.